The present invention relates to a battery cooling device.
More particularly, the present invention provides an efficient electrolyte circulation and cooling device which is suitable for dissipating heat from a metal-air battery intended for vehicle propulsion.
All electric batteries generate heat during operation, one source thereof being the chemical reaction taking place in the cells, and the second source being the current flow meeting the ohmic resistance of the battery itself. In most batteries, such heat is dissipated naturally, mainly by convection. However, batteries designed to power electric vehicles are designed to provide high power from a compact battery envelope, and may exhibit a high temperature rise if no additional provision is made for cooling. Further aggravating the problem is the consideration that road vehicles need to be designed to allow operation in ambient temperatures as high as 45.degree.-50.degree. C.; under such conditions, the generated heat may raise battery temperature to a level causing irreversible battery damage.
While a high operating temperature is not necessarily detrimental to battery performance, there are several reasons justifying some cooling arrangement in a battery of this type. First, in a naturally cooled battery, the inner cells will operate at a considerably higher temperature than those on the battery perimeter, leading to operating differentials between the cells. Second, the temperature may rise above that allowed for the plastic parts of the cell, or for the electrolyte. Third, it is often advantageous to use the heat removed from the battery for heating the passenger compartment of the vehicle, when required.
Cooling systems for electric batteries are described in U.S. Pat. Nos. 754,969; 3,767,468; 4,007,315; 4,463,064; 4,578,324; 4,925,744; and 5,212,024. These specifications disclose various systems for circulating a cooling gas such as air, or a liquid such as water, through the battery for removing heat therefrom.
As metal-air batteries contain an electrolyte in liquid form, such batteries can conveniently be cooled by circulating this electrolyte through some form of cooling system.
With regard to a cooling system for a battery intended for use for the propulsion of automobiles and vans, it is important that the weight, and to some extent, also the bulk of such a system, be kept to a minimum, due to the fact that the battery and its auxilliaries should be as lightweight and Compact as possible, for given vehicle/performance requirements.
In a cooling system, the heat transfer coefficient achieved in a tube coil or heat exchanger is directly proportional to the temperature difference between the cooling fluid or gas and the liquid being cooled, in the present case, the electrolyte. It follows that a compact, light-weight system can be constructed where it can be arranged that such a temperature differential is high. A practical way of achieving this end is to circulate the hot electrolyte from the cells into a small, separate compartment and there to transfer some of its heat to the cooling fluid passing therethrough for external dissipation.
It is therefore one of the objectives of the present invention to obviate the disadvantages of the prior art electrolyte cooling devices, and to provide a cooling device which will efficiently achieve its function with the addition to the battery of only minimal weight and bulk.
The present invention achieves said objective by providing an electrolyte cooling device for use in combination with a multi-cell metal-air battery, said device comprising an electrolyte reservoir divided into a smaller first compartment and a larger second compartment, said compartments being connected by a valve arranged to periodically allow flow of electrolyte from said smaller compartment to said larger compartment; a warm electrolyte liquid flow path leading from an electrolyte-holding volume of each cell of said battery to an inlet port of said smaller compartment; a cooled electrolyte liquid flow path leading from an outlet port of said larger compartment to said electrolyte-holding volume of each said cell; at least one cooling fluid conduit passing through said smaller compartment; and pumping means for circulating a cooling fluid through said conduit and through radiator means arranged for the disposal therefrom of heat.
In a preferred embodiment of the present invention, there is provided an electrolyte cooling device wherein said larger compartment comprises a gas-tight vessel in fluid communication with air compression means for effecting circulation of said electrolyte.
Preferably, said air compression means comprises an inlet in said larger compartment for supplying compressed air thereto, whereby, after closure of said valve, increased air pressure in said larger compartment drives electrolyte therefrom into each said cell.
In a most preferred embodiment of the present invention, there is provided an electrolyte cooling device wherein said hot electrolyte liquid flow path is in the form of an unequal-leg inverse U, the upper bend of said U being a small tank exposed to air pressure in the cell, whereby said flow path is arranged for entry of hot electrolyte from said electrolyte-holding volume of each cell of said battery into the open lower portion of the shorter leg of said U, said electrolyte being pressured to flow upwards into said small tank, from which said electrolyte enters the upper opening of the longer leg of said U and flows down said longer leg under gravity to enter said smaller compartment through said inlet port.
A metal-air battery and an electrolyte circulation system used for the cooling thereof is described and claimed in U.S. Pat. No. 5,093,213. The system described therein includes an electrolyte reservoir which is divided into compartments by a divider wall, the top of said divider wall forming an overflow weir. The aim is to allow liquid electrolyte to flow over the weir but to retain solid waste products, such as metal hydroxide, for later removal from the bottom of the reservoir. The division of the reservoir into compartments by said dividing wall is only up to part of the height of the reservoir, the weir providing overflow from one compartment to the other. Furthermore, the divider wall of said patent is not used to improve cooling.
Said patent also envisages the use of one or several electric pumps for achieving circulation, and electrically-driven air fans are provided to cool the heat exchanger. An auxiliary battery is required for starting. The device described in said patent is quite complex, with attendant cost, weight and reliability penalties.
A further system provided with a divided reservoir is described in U.S. Pat. No. 3,290,176. In this device, three electrolyte reservoir compartments are formed by two partitions, but these partitions have a small lower space interconnecting the compartments. Thus, electrolyte may flow under said partitions in either direction, and therefore the device of this patent does not provide for the efficient, batch-like cooling of aliquot portions of electrolyte, as provided by the system of the present invention.
The invention will be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.
With reference to the figures, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.